This invention relates to a data transmission system in which the data to be transmitted is expressed in a binary code which is converted into a ternary line code before transmission.
An example of such a ternary code is described in U.S. Pat. No. 3,611,141 (D. B. Waters), where the code is of the 4B3T type, which means that successive groups each of four binary elements (or bits) are converted for transmission into groups of three ternary elements. Another example of such a code is the equally well-known 3B2T line code.
British Published Application No. 2014401A (Hitachi) describes a data transmission system in which a line code for transmitting intelligence in binary form is described, and which uses three different line levels. Thus it is a pseudo-ternary line code. Two of the levels are used to convey the two binary conditions, 1 and 0, while the third level, referred to as level .0., is used as a "repeat indicator". Thus if a succession of 1 digits have to be sent, they are sent as 1, .0., 1, .0. . . . ; similarly if a succession of 0 digits have to be sent, they are sent as 0, .0., 0, .0., . . . What this means is that a succession of digits of the same binary signification is conveyed as an alternacy of the line level for that binary signification and the third (.0.), or "repeat indicating" level.
It will be noted that in this system the number of three level, or pseudo-ternary, elements which are sent is the same as the number of binary elements in the original message.
Another line code of some interest is described in U.S. Pat. No. 3,418,631 (Sipress et al), in which the input signal is split into two-bit binary words and then encoded for transmission into three-level signals according to either of two codes. This is called a specially constrained code, and is also described in U.S. Pat. No. 3,302,193 (Sipress). U.S. Pat. No. 4,502,143 (Kato et al) describes a line code arrangement in which the object is to reduce the number of consecutive identical digits to a reasonably low level. In addition, a number of other known line codes are described. A final document of interest is U.S. Pat. No. 4,520,346 (Shimoda) which relates to an NRZI (non-return to zero inverted) code format.